Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Insights The Vacuum Fluctuation Myth - Comments

  1. Jan 14, 2017 #201
    http://wikidiff.com/quantum/discrete

    I think I used quantum/quantized in the appropriate way.
    Discrete things do not have to be of the same size. But quantum things (quanta) are supposed to be of the same size, like fundamental units or fundamental blocks. Because of that, they are like fundamental deltas or units of change.
     
  2. Jan 14, 2017 #202
    https://en.m.wikipedia.org/wiki/Quantum

    In physics, a quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction. The fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization".[1] This means that the magnitude of the physical property can take on only certain discrete values

    How can you possibly apply the above to a VP ?
     
    Last edited: Jan 14, 2017
  3. Jun 22, 2017 #203
    Some people still seem to think vacuum fluctuations are a real thing.
     
  4. Jun 22, 2017 #204

    vanhees71

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Yes, it's very hard to get wrong ideas from popular-science books (sometimes even textbooks!) out of the minds of people. Some famous guy (Feynman?) said, that for any problem there's a simple solution, which is wrong.
     
  5. Jun 22, 2017 #205

    mfb

    User Avatar
    2017 Award

    Staff: Mentor

    I don't see where the second quote would imply vacuum fluctuations. We cannot really predict a cosmological constant from QFT, but if it is non-zero, the natural scale would be the Planck density.
     
  6. Jun 22, 2017 #206
    I have a closely related question. In the insight it is mentioned that generally, inside a superposition nothing dynamical happens. But what if the wave function is time dependent, for instance an electron in the double slit experiment, do fluctuations in the quantum state happen then?
     
  7. Jun 23, 2017 #207

    A. Neumaier

    User Avatar
    Science Advisor

    In this case the wave function changes deterministically. The wave function of the spin of an electron changes very smoothly with time, except at the moment of measurement, where it changes randomly. But this randomness has nothing to do with vacuum fluctuations.
     
  8. Jun 23, 2017 #208
    But would you still say that nothing dynamical happens inside the wavefunction before the measurement, even if we take into account the deterministic evolution you mentioned?

    It's not like the electron is jumping from one spot to anothee. It literally is in a state without a definite position, right?
     
  9. Jun 23, 2017 #209

    vanhees71

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The wave functions evolve according to Schrödinger's equation (i.e., unitary time evolution). I don't know what you mean by "nothing happens inside the wave function". The Schrödinger equation is describing the dynamics of the system.

    According to quantum theory nothing is jumping at all. It's another bad idea from "old quantum theory" that should not be used in any modern physics curriculum anymore. Indeed an electron has never a definite position (although in principle it can be quite localized, because it's a massive particle and thus admits the definition of position as an observable). Within non-relatistic quantum theory the position-probability distribution is given by ##|\psi(t,\vec{x})|^2##.
     
  10. Jun 23, 2017 #210

    A. Neumaier

    User Avatar
    Science Advisor

    The smooth evolution of the wave function is dynamically happening while it passes a magnet. It is not the wave function but the intuitive semiclassical picture of an electron as a moving point that ''causes'' the apparent jumps.

    To say that the electron has no definite position just means that one cannot think of it as being a point. The position of an electron is as well-defined as that of a cloud - it is located in a well-defined region but not in a well-defined point. Only the latter would have a definite position. Thus if one wants a more valid intuitive picture one needs to consider an electron as a smoothly changing cloud distributed over all electron rays with a non-negligible mass density, and contracting to a small spot upon measuring.
     
  11. Jun 23, 2017 #211
    Got it. To me, to say that the electron ia fluctuating in its position is roughly to say that it is jumping between one position and another when not measured which is clearly related to the semiclassical picture you (and vanhees) described. And which is of course false.

    When I said nothing happens in the wf, I meant that. Electron isn't jumping between the spots, it literally is in a state of variance which may change over time. But it is still only a variance - nothing happens 'inside' the wf. This is a different context of happening than deterministically evolving which applies to the wf as a whole. For something to happen, you need measurement. Would you agree with this line of reasoning?
     
  12. Jun 23, 2017 #212

    A. Neumaier

    User Avatar
    Science Advisor

    No. In more complicated contexts, a lot may happen, and this is expressed in the evolution of the state. Indeed, ''the moment of measurement'' is itself a gross simplification of a very complicated interaction that happens between the electron and the measurement device, described not by the state of the electron alone but by the state of the combined system electron+device+environment. The apparent randomness in the fate of the electron state alone is due to this additional complexity.
     
  13. Dec 7, 2017 #213
    I used to know a bit about the "toy model" ##\phi^4_2## QFT in 1+1 dimensions (Glimm & Jaffe). They rigorously construct interacting particle states with bound states and scattering. AFAIK there is nothing corresponding to vacuum fluctuations in this mathematically well defined theory. I can't think how you could even rigorously ask "are there vacuum fluctuations?" in this context.
    "Vacuum fluctuations" seem to be an artifice of trying to apply perturbation theory when you don't know that the perturbed theory is mathematically well defined.
     
  14. Dec 8, 2017 #214

    A. Neumaier

    User Avatar
    Science Advisor

    Vacuum fluctuations refer to the fact that smeared field operators have in the vacuum state a nonzero variance. This is captured by the Wightman distribution functions, hence a fact even in ##\phi^4_2## QFT in 1+1 dimensions.

    On the other hand, interpreting (as in most popular accounts of quantum phenomena) these vacuum fluctuations as happenings in time is completely fictitious.
     
  15. Dec 8, 2017 #215
    If vacuum fluctuations are fictitious, then what is the proper explanation of the Casimir effect?
     
  16. Dec 8, 2017 #216

    A. Neumaier

    User Avatar
    Science Advisor

  17. Dec 8, 2017 #217

    bhobba

    Staff: Mentor

    Its actually a manifestation of Van Der Walls forces you probably learned about in HS chemistry::
    https://arxiv.org/pdf/hep-th/0503158v1.pdf
    'The Casimir effect is a function of the fine structure constant and vanishes as α → 0. Explicit dependence on α is absent from eq. (3) because it is an asymptotic form, exact in the α → ∞ limit. The Casimir force is simply the (relativistic, retarded) van der Waals force between the metal plates'

    See also:
    https://arxiv.org/abs/1605.04143

    Thanks
    Bill
     
  18. Dec 9, 2017 #218
    Thank you Dr. Neumaier and Dr. Bill for taking the time to respond to my post. I have been perplexed and annoyed by the term "vacuum fluctuation" for many years. After reading the paper by Jaffe my angst has been lifted and I have a renewed appreciation for the incredible predictive power of QFT. Alhamdu lila!
    Salam,
    Fred
     
  19. Dec 9, 2017 #219

    jtbell

    User Avatar

    Staff: Mentor

    The source of this (mis)interpretation is surely the fact that in normal English usage, the word "fluctuation" does mean "variation with respect to time." At least, that's how I always understand it in everyday language.

    This is of course not the only case in people are confused by the re-purposing of everyday words into physics jargon with specific technical meaning. Consider "work", "energy", and "power", which introductory physics students often struggle with at first. Or "speed" and "velocity".
     
    Last edited: Dec 9, 2017
  20. Dec 9, 2017 #220
    The word vacuum is also problematic when you look at the definitions from classical to quantum.
     
  21. Dec 9, 2017 #221

    bhobba

    Staff: Mentor

    Many that post here have doctorates, including Dr. Neumaier. I however am not one. Just a guy with a degree in applied math and computing who is now retired from 30 years spent programming, so can indulge his fascination for and interest in physics.

    Thanks
    Bill
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted